I'm working on a SD 4:30 project and wasn't prepared to touch HD anything but an accident happened. I wound up needing 10 minutes worth of footage that was accidently shot by a Sony HC3 camera in HD mode using regular mini-dv tape in 16:9.

I think I captured it ok in FCP6 by changing the sequence and capture presets. It plays on the computer anyway.

So how do I get the sequence so that I can use it in my SD 4:3 project?

You might want to have a play with the aspect ratio slider in the distort feature in the motion tab of the viewer window.
Once you've got one frame right you can cop[y/paste the attributes into the other clips. You're going to lose some information but it should look ok.

Simply drop your HD footage into your SD timeline, FCP6 will sort out the correct aspect ratio. You will have to use the motion tab if you don't what a letterbox though, just scale it up and pan-and scan.

This is correct. FCP will convert it to the proper aspect ratio but letterboxed. I've done the same thing--ie., used HDV footage in an SD 4:3 timeline. The HD footage is high enough resolution compared to SD that you can use the motion tab to blow it up easily if you want to get rid of the letterbox, and it'll still look great.